A wire cut electrical discharge machining apparatus is an apparatus designed to use a thin wire of about 0.05 to 0.35 mm.phi. in diameter as an electrode tool. The wire is held in tension between a pair of positioning guide members which permit renewal of the wire electrode in the work zone by allowing movement of the electrode wire in its axial direction while maintaining a predetermined tension on it. During normal machining, the wire electrode approaches the workpiece to be machined at approximately a right angle with respect to the axis of the wire electrode, while maintaining a very small machining gap between the workpiece and the wire electrode. Machining is done by generating spark discharges on an intermittent basis. The discharges are generated by applying periodic voltage pulses across the working gap between the wire electrode and the workpiece under conditions where a working fluid fills the working gap. In such an apparatus, the machining feed device causes relative movement between the wire electrode and the workpiece as machining progresses. Unless otherwise prevented by suitable machining condition settings and control of working conditions, machining may be interrupted as a result of wire electrode breakage. Some recent arrangements for dealing with wire electrode breakage include self-recovery devices, such as, for example, a function which automatically joins broken wire electrode ends or otherwise rethreads the electrode through the workpiece. Such joining or rethreading functions are not necessarily 100 percent successful and, in any event, even if successfully accomplished, are not only wasteful of time, but also generally impair machining accuracy and the like. It is, therefore, preferable to avoid wire breakage. However, to do so in the context of prior art wire cut electrical discharge machines creates other problems. In order to machine accurately and with high performance, i.e., efficiently and at higher machining speed, working conditions, and in particular machining voltage pulse width and off time duration, mean amplitude of the discharge current, and machining feed control, must be established and controlled so that the machining is done under heavier load conditions at which there is a higher risk of wire breakage. Moreover, it is impossible to carry out precision machining wherein the wire electrode is highly tensioned without risking wire breakage, unless the machining is done at relatively low efficiency working conditions.
There are various causes of wire electrode breakage. However, wire breakage during machining may, to some extent, be avoided by detecting spark conditions, and controlling the working conditions to light-load conditions if a concentration of spark positions is detected; in other words, a condition where the sparks are only partially distributed along a limited area of the machining gap. Wire electrode breakage due to causes such as spark concentration may be avoided through the use of gap condition detection and adaptive control based on that detection. Appropriate detection, for the purpose of preventing wire electrode breakage, requires detecting the position of the sparks being generated within the working segment of the wire electrode, i.e., the length of wire electrode between the electrical feed or contact pieces positioned on both the upper and lower sides of the workpiece, and especially along the spark-generating portion of the wire electrode, i.e., the axial portion of the wire electrode which defines the machining gap between the wire electrode and the workpiece, and varyingly and responsively controlling the working conditions in accordance with the positions at which spark discharges are generated each time a machining pulse, in the form of a voltage impressed across the machining gap, is generated or in accordance with the position of a sampling of spark discharges from each machining pulse. The detection should be able to determine whether the spark discharge positions are concentrated or not to thereby determine whether conditions which may result in breakage of the wire electrode are present. In other words, the detection should be capable of discriminating whether the spark discharge positions are concentrated or only partially distributed along the wire electrode. Such detection may involve detection of spark discharge positions generated within a predetermined period of time or within predetermined numbers of spark pulses.
Most known detectors of concentrated spark conditions for conventional wire cut electrical discharge machining apparatus indirectly estimate the spark discharge positions according to parameters, such as different spark conditions, in an effort to discriminate a concentrated spark discharge situation from a normal one. For example, according to Laid-Open Japanese Patent Application No. 53-64899, spark discharge positions are detected or measured in an essentially indirect manner by providing a voltage measuring circuit in which fluctuations in electric resistance between an electrical contact point (of the wire electrode) and the sparking point is determined by measuring the voltage therebetween.
Further, according to Laid-Open Japanese Patent Application No. 59-30620, wire breakage prevention is attempted by detecting the value of the current flowing into the upper and/or lower electrical feed or contact pieces, extracting a signal indicative of a changing rate of current flowing into the feed contact pieces from the current detection signal, comparing the current rate change with a predetermined value in order to attempt to determine conditions wherein sparks are concentrated at or nearly the same location on the workpiece, and then resetting the working conditions in response to the detected signal. However, in accordance with this technique, the correlation between the moving speed of the sparking position on the wire electrode and the wire traveling speed is not taken into account and, therefore, the detection of a concentrated sparking condition at the same point on the wire electrode is not possible. In Laid-Open Japanese Patent Application No. 62-54626, the moving speed of the sparking position on the wire electrode and the traveling speed of the wire electrode are detected and when both speeds are almost coincident for more than the predetermined period of time, a concentrated sparking condition detection signal is delivered.
According to the above-stated disclosures and the like, even if the spark discharge positions could be detected or measured with some accuracy, questions remain about spark concentration, methods of discriminating between partially distributed and concentrated spark discharge positions, determination of criterion for discrimination, and the like. Therefore, the present state of the art is not yet effective for practical use.
For example, in the '620 Laid-Open Application the arrangement is such that a spark discharge position signal is detected for every voltage pulse (specifically, the machining current in the electrical feed or contact piece circuit is detected while machining voltage pulses are in a gate-off condition, the value held in a sample-hold circuit, and the value taken as a digital value using an A/D converter), and is compared with the value of the previous spark pulse to detect any difference (by using an arithmetic unit or digital comparator) and, if there is a difference, a difference signal is delivered. Every time the spark discharge position signal is output, the output is integrally counted. When a predetermined set number n is counted, the count of the counter which delivers a varying control signal for setting working conditions, is reset by the difference signal. The count is set using, for example, a preset counter. Conditions for difference detection and the values used, such as the number of times the difference detection delivers the reset signal, is carried out and the set number n for the said preset counter can be freely regulated and set by experience, data and theory, according to selected, predetermined working conditions and machining objectives. However, the machining efficiency is poor under settings which usually overemphasize safety, while smaller safety margins may result in accidental breakage of the wire electrode. Therefore, in practice, operation at preferable settings adapted to individual working conditions are not attained, and the resulting situation is impractical.
In addition, in practical usage, the spark discharge position detection apparatus according to the '620 Laid-Open Application, does not detect spark position with satisfactory accuracy levels.
Similarly, the use of means, such as a difference amplifier, as disclosed in Laid-Open Japanese Patent Application No. 62-15017, for spark discharge position detection does not provide sufficient accuracy as is needed to effectively prevent wire electrode breakage without adversely affecting other aspects of machining performance.
Also, with regard to the disclosure of the '626 Laid-Open Application, if there is a concentration of sparks attributable to chips and/or other foreign matter which has adhered to or become fixedly stuck on the surface of the wire electrode, it can be detected as is disclosed. However, in practice, such a situation is not usually a problem encountered during machining. Today, when machining under usual working conditions, the situation where the traveling speed of the wire electrode has to be used as a parameter for control is unusual. For example, machining is typically carried out using an electrode renewal feed rate of about 3 to 8 m/min for a brass wire electrode of between about 0.2 and 0.3 mm in diameter. Of course, this feed rate may vary depending on the wire electrode material and diameter and other working conditions. Different feed rates and wire electrode material may be used. Assuming conditions such as voltage pulse ON time of about 1 .mu.s or less, about 4 to 10 .mu.s of quiescent (or off) time between sparking pulses, a sparking current of about 10 to 15A, and a machining speed of between about 100 and 170 mm.sup.2 /min, then about 1000 voltage pulses are supplied and impressed between the wire electrode and the workpiece while the wire electrode moves by about 1 mm. Therefore, the appropriate detection strategy is to first detect the spark concentration conditions at locations adjacent to the initial spark discharge position, as opposed to tracing the spark discharge positions on the wire electrode, and then carrying out discrimination. In addition, if the period of concentrated spark duration times is in the range of 50 ms to several 100 ms, as discussed in the '626 Laid-Open Application, the wire electrode would move axially by between about 5 mm and several 10 mm in the time frame of interest. However, the number of machining voltage pulses detected in that time frame (to be used as discrimination data) will be approximately between 50,000 to several 100,000 pulses. From a practical standpoint, it would be difficult to use such a number as a set count number for the counter 34 and/or to determine a suitable frequency setting of the clock pulse generator 3 corresponding to predetermined wire cut spark machining conditions.
The present invention is based in part on a recognition that the spark discharge conditions in a wire cut electrical discharge machine during various machining modes, such as the advancing mode in machining, is roughly as follows.
The wire electrode moves on a renewing feed basis between a pair of contacted positioning guides, while experiencing oscillations (sometimes including superimposed higher oscillations) whose characteristics depend on such parameters as the material, diameter, distance between guides, wire tension, etc. Other oscillations, which are restricted by machining conditions imposed on the traveling portion of wire electrode, e.g., such as where the wire passes through the narrow groove inside which the working fluid is positioned, also exist. Under conditions where intermittent voltage pulses are impressed across the machining gap between the wire electrode and the workpiece, as indicated above, with the wire approaching the workpiece at machining speeds as indicated above, an intermittent sparking pulse of approximately 10 kHz to several 10 kHz on the average is initiated across the machining gap, e.g., where the wire and workpiece are in closest proximity or where foreign matter causes light spark conditions, giving rise to the characteristic EDM sparking. If everything is operating normally, the spark discharges resulting from those sparking pulses move at random along the axial direction of the wire electrode, i.e., along the thickness of the workpiece, as machining progresses. The workpiece is thus subjected to even and equally distributed spark discharges. When sparks start to occur at the same proximate location as a preceding spark, after some time interval additional chips, gas, bubbles, and the like, intervene around the sparking point as a result of the sparks. Assuming an arbitrary number of voltage pulses successively supplied and impressed across the machining gap at a frequency of up to approximately 100 kHz (e.g., using the energy formula disclosed in Laid-Open Japanese Patent Application No. 44-13195, and assuming a spark discharge takes place almost at the instant the voltage pulses are initiated, the sparking frequency and machining pulse frequency will be almost the same), usually several pulses will strike a succession of sparks at or near the same sparking point (of course, not all successive voltage pulses will strike sparks in other (i.e., distributed) locations, and some voltage pulses create no sparks at all). A condition whereby a slight concentration of sparks at or near one location may develop.
Oscillations of the wire electrode may be present from other causes. However, the gap is more likely to expand locally due to influences such as the pressure from the spark discharges and the produced gas, chips and the like which tend to diffuse and move to the circumference. As a result, as described above, most spark discharges take place in the gap at other locations and, as described above, as machining is carried out, the spark discharges move tend to to other locations immediately or after striking some arbitrary number of sparks.
Therefore, it is difficult to discriminate an undue spark concentration conditions based on the exact position of the detection of spark discharges and the procedure of processing multiple spark discharge position detection signals. The above description concerns the case where the machining gap and machining conditions are normal.
As alluded to above, detection of concentrated spark discharge conditions avoids problems. It should be appreciated that there are some cases where the cause is unknown and concentrated sparks cannot be detected. However, situations such as where the machining feed rate increases beyond the normal rate or where the workpiece is thicker in comparison to machining under the normal spark condition as described above, or where there is a deficient or unsuitable supply of working fluid in the gap due to a change in taper angle or when machining around a corner part of a profile configuration to be machined, or where foreign matter or the like adheres to the wire electrode or a local part of the workpiece, or where an overheated spot is created, all may lead to breakage of the wire electrode. Other causes may be a location in the gap wherein several sparks occur and which is not relieved of the arc-like discharges but instead develops short circuit, or where sparks are produced as a result of a machining spark phenomena in a certain location, which, for some reason becomes arc-like. Then, sparks in that location may continuously repeat for a period several times longer than that associated with the slight concentration of sparks which occurs during normal machining. In some cases, the result is an arc-like concentration of sparks leading to breakage of the wire electrode.